Dr Karen Beckerman, an obstetrician from the Bay Area Perinatal Aids Center in San Francisco, says none of the 86 women she has treated has transmitted HIV to their babies.

She says she gives women up-to-date information about drugs available to treat HIV and promotes the use of combination therapy of at least three anti-HIV drugs throughout pregnancy.

In the UK, not many mothers-to-be use combination therapy.

The Aids Treatment Project says this is because AZT is currently the only approved drug for pregnant women in the UK.

"We are lagging behind the USA," said a spokeswoman.

Aids charity Avert says the problem is that, apart from AZT, most anti-HIV drugs have not been around for very long and scientists are still learning what effects most might have on unborn babies.

It adds that there are indications that some may have bad effects on pregnancy so women should discuss their medication with their doctor if they are pregnant or planning a pregnancy.

And it recommends that, if the woman's health is relatively good, she should steer clear of drugs in the first 14 weeks when the baby is most at risk of developing defects.

Target group

Pregnant women are considered one of the key target groups in the government's Aids strategy.

On World Aids Day last year, it highlighted the fact that anonymous research in ante-natal clinics, particularly in London, showed many women were HIV positive without knowing it.

The government says it is important that pregnant women be offered HIV tests so they can take action to prevent the virus being passed to their babies.

Studies show that the number of babies born with HIV fell by two thirds if women took AZT during pregnancy and did not breastfeed.

Dr Beckerman believes this number can be further reduced if combination therapy is used.